This invention relates to molten metal such as molten aluminum, and more particularly, it relates to an improved method for dispersing treatment media such a fluxing gas or salts in molten metal.
In the prior methods of dispersing fluxing gas, for example, in a molten aluminum body, the fluxing gas is introduced down a shaft into the body and dispersed by a rotating impeller mounted on the shaft. However, this method is not without problems. The rotating impeller creates a vortex about the shaft that indicates that a large portion of the molten metal is swirling or circulating about the impeller shaft at a rate approaching the rotation speed of the impeller. Fluxing media added to the molten metal tends to circulate with the molten metal with only minimal dispersion. Further, the vortex has the effect of increasing the surface area of the molten body exposed to air. The increased exposure of the molten metal to air results in an increase in dross formation, subsequent entrainment of the dross and its detrimental collateral effects. When the fluxing material is a gas, the vortex creates a problem in yet another way. Fluxing gas is displaced towards the center of the vortex by body force separation with the result that other parts of the molten body are not adequately treated with fluxing gas. Thus, the effectiveness of the process is reduced because portions of the molten body do not get treated with fluxing material. In addition, fluxing gas entrained in the molten metal flow pattern tends to coalesce, resulting in larger bubbles of fluxing gas developing in the melt. The larger bubbles lower the effectiveness of the fluxing process because less molten metal gets treated.
Common methods employed to suppress vortex formation include the insertion of baffles or rods into the melt. However, baffles are undesirable because a dead volume develops behind the trailing edges of the baffle. Another method used to suppress vortex formation is to limit power input to the impeller. However, this severely limits efficiency.
These problems continue to plague the industry as indicated in U.S. Pat. No. 5,160,693, for example, which discloses that with rotating impellers a surface vortex forms, the vortex rotating about and flowing downwardly along the impeller shaft, thereby agitating surface dross and drawing impurities back into the melt. The patent also indicates that an ideal system would minimize disturbances to the surface dross to prevent recontamination of the treated melt.
Thus, there is a great need for a more effective fluxing process which suppresses ingestion of dross from the surface back into the melt by vortex formation, for example, maintains the fluxing material finely dispersed throughout the molten body, and intensifies the contact of molten metal with fluxing material for improved fluxing of the melt.